For those who don't trade on margin, the existence of margin trading tends to exaggerate the peaks and troughs of the market. When prices are rising, it greatly increases the number of BTC that can be bought, causing the price to rise further. When prices are falling fast enough to force liquidations, it greatly increases the number of BTC that are being sold, which depresses the price further. I don't mind this, because it gives me more opportunity to buy very low and sell very high.